Cellulose-based nano-sized fibrils provide new possibilities for producing light and strong materials. For example increasing environmental requirements promote more extensive utilization of new natural fiber based biomaterials in the future. Nanosized materials can provide properties which can not be achieved which larger sized particles. The smaller the particle, the larger the surface area is and more possibilities for desired interactions with other materials exist.
Cellulose fibers (width 30-40 μm, length 2-3 mm) can be dismantled into nanosized structures (width about 5-30 nm, length several μms). Microfibrillated cellulose (MFC) has been produced by combining enzymatic or chemical treatments to mechanical treatments. Microfibrils provide even in minor proportion conventional paper products increased toughness and strength. International patent publication WO 2007/091942 discloses a method for manufacturing microfibrillated cellulose using enzymatic treatment.
Properties of the cellulose fibers used for producing paper can be modified by adding polymers to the fiber suspension. Suitable additive polymers include for example starch-based polymers, such as cationized starch, or synthetic polymers such as polyacryl polymers, polyamineamide-, polyamine- and acrylamino-epichlorohydrine polymers, cellulose derivatives or anionic polymers containing carboxyl groups or carboxylate ions in the form of alkali metals of ammonium salts, for example carboxymethyl polysaccharides, such as carboxymethyl cellulose (CMC). International patent publications WO 01/66600 and WO 00/47628 disclose derivatized microfibrillar polysaccharides, such as cellulose and production methods thereof.
CMC or sodium carboxymethyl cellulose is a water-soluble anionic polymer achieved by introducing carboxymethyl groups along the cellulose chain. The functional properties of CMC depend on the degree of substitution on the cellulose structure (i.e. how many of the hydroxyl groups have taken part in the substitution reaction), and also on the chain length of the cellulose backbone. The degree of substitution (DS) of CMC is usually in the range from 0.6 to 0.95 derivatives per monomer unit.
CMC can be used as an additive during the grinding of paper pulp (B. T. Hofreiter in “Pulp and Paper Chemistry and Chemical Technology”, Chapter 14, Volume III, 3rd. edition, New York, 1981; W. F. Reynolds in “Dry strength additives”, Atlanta 1980; D. Eklund and T. Lindstrom in “Paper Chemistry—an introduction”, Grankulla, Finland 1991; J. C. Roberts in “Paper Chemistry”; Glasgow and London 1991).
CMC has a low affinity for cellulose fibers, since both are anionically charged. CMC can still be attached irreversibly to pulp fibres and it increases the surface charge density of pulp fibres.
U.S. Pat. Nos. 5,061,346 and 5,316,623 disclose the addition of CMC to pulp in paper making processes. Publications WO 2004/055268 and WO 2004/055267 present fiber suspensions comprising cellulose enzyme-treated microfibrillar sulphate pulp (eMFC) and carboxymethyl cellulose (CMC) as raw material for packages and for surface application in paperboard and paper production, respectively.
CMC is used as thickener to modify the rheology. CMC has also been used as a dispersion agent. Furthermore, CMC has been used as binder. U.S. Pat. No. 5,487,419 discloses CMC as dispersion agent. U.S. Pat. No. 6,224,663 discloses use of CMC as an additive in a cellulose composition. Publication WO 95/02966 discloses the use of CMC to modify microcrystalline cellulose and in some cases microfibrillated MCC by mixing the two components and the use of this mixture in food compositions.
CMC sorption is known in the art. U.S. Pat. No. 6,958,108 and international patent publication WO 99/57370 disclose a method for producing a fiber product, wherein alkali soluble CMC is added to the pulp under alkali conditions. International patent publication WO 01/021890 discloses a method for modifying cellulose fibers with a cellulose derivative such as CMC. Publication WO 2009/126106 relates to attachment of amphoteric CMC polymers to cellulose fibres before homogenization.
The following articles by Laine et al. disclose modification of cellulosic fibers with CMC: Nord Pulp Pap Res J, 15:520-526 (2000); Nord Pulp Pap Res J, 17:50-56 (2002); Nord Pulp Pap Res J, 17:57-60 (2002); Nord Pulp Pap Res J, 18:316-325 (2003); Nord Pulp Pap Res J, 18:325-332 (2003).
Despite the ongoing research and development in the manufacturing of microfibrillated cellulose there is still a continuing need in the industry to Improve the processes. One problem is high energy consumption and thus there is a need for an energy efficient method. There is also a need for a process, wherein the properties of paper are improved. The present invention provides a method for overcoming the problems associated with the prior art.